LT1573
APPLICATIO S I FOR ATIO
The LT1573 is designed to be used in conjunction with an
external PNP transistor. The overall specifications of a
regulator circuit using the LT1573 and an external PNP will
be heavily dependent on the specifications of the external
PNP. While there are a wide variety of PNP transistors
available that can be used with the LT1573, the specifica-
tions given in a typical transistor data sheet are of little use
in determining overall circuit performance. In the follow-
ing discussion the critical requirements of the PNP tran-
sistors are noted. Design equations are given and
examples are shown using a readily available discrete PNP
transistor. This device is inexpensive, available from mul-
tiple sources and can be used for a wide range of applica-
tions. For applications using other PNP transistors, the
regulator specifications can be derived by the same method.
Basic Regulator Circuit
The basic regulator circuit is shown in Figure 2. The
adjustable output LT1573 senses the regulator output
voltage from its feedback pin via the output voltage
divider, R1 and R2, and drives the base of the external PNP
transistor to maintain the regulator output at the desired
value. For fixed output versions of the LT1573, the regu-
lator output voltage is sensed from the feedback pin via an
internal voltage divider. The resistor R
D
is required for the
overcurrent latch-off function. R
D
is also used to limit the
drive current available to the external PNP transistor and
to limit the power dissipation in the LT1573. Limiting the
drive current to the external PNP transistor will limit the
output current of the regulator which minimizes the stress
C
C
FB
LATCH
COMP
V
OUT
V
IN
R
D
GND
V
IN
DRIVE
+
C
TIME
LT1573
SHDN
R
B
Q
OUT
V
OUT
R1
Figure 2. Basic Regulator Circuit
U
on the regulator circuit under overload conditions. The
resistor R
D
is chosen based on the operating requirements
of the circuit, primarily the dropout voltage and the output
current. The dropout voltage of an LT1573-based regula-
tor circuit is determined by the V
CE
saturation voltage of
the discrete external PNP transistor when it is driven with
a base current equal to the available drive current of the
LT1573.
External PNP Transistor Selection Criteria
The selection of an appropriate external PNP transistor
depends on the regulator application specifications. The
critical PNP transistor selection criteria include:
1. The maximum output current of the PNP transistor
2. The dropout voltage at the maximum output current
3. The gain-bandwidth product f
T
of the transistor
The PNP transistor must be able to supply the specified
maximum regulator output current to be qualified for the
regulator application. The V
CE
saturation voltage of the
transistor at the maximum output current determines the
dropout voltage of the circuit. The dropout voltage deter-
mines the minimum regulator input voltage for a certain
specified output voltage. The gain-bandwidth product f
T
of the transistor determines how fast the voltage regulator
can follow an output load change without losing voltage
regulation.
The D45H11 from Motorola and the KSE45H11TU from
Samsung can be used in all LT1573 regulator circuits with
current ratings up to 5A. The D45H11 can supply 5A of
R
C
W
U U
+
C
IN
C
OUT1
+
C
OUT2
R2
LOAD
GND
1573 F02
7
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